1. Field of the Invention
The present invention relates to a computer program for calculating the shear and axial forces on fasteners used to form structural joints and, more particularly, for predicting the highest force that will be applied to any of the fasteners comprising a joint when the structure is loaded, and for identifying the fastener that will transmit the foregoing highest force.
2. Description of the Prior Art
Structures are commonly constructed by using joints comprised of fasteners to attach load-bearing members such as beams and bars, to each other and to various functional components. The members are subjected to axial forces, shear forces and moments. In a simple structure, the foregoing parameters may be manually calculated by a structural engineer using well known analytical methods. However, such calculations are very time consuming when the structure is complex, or when the applied forces are dynamic and numerous cases must be considered during the design process.
Computer programs for use in conjunction with digital computers have been developed as an analytical aid for use in such situations. The NASTRAN computer program in particular has been found to be a valuable tool in the analysis of complex structures such as those used in flight vehicles. This program is well known and widely available to those practicing in the field of structural analysis.
The NASTRAN program allows the design engineer to subject the structure to a number of different load states, also known as subcases, and obtain the axial and shear forces and moment acting on each location of interest, also known as a grid point. The axial and shear forces and moments acting on the structural members are then derived from the forces and moments at the relevant grid points. For example, the NASTRAN program has been used to calculate the forces acting on a number of grid points on a flight vehicle for subcases consisting of different inertial loads. It also enables the designer to ascertain the respective changes in the forces at grid points when the structure is modified.
Moreover, the NASTRAN program can determine the forces acting on grid points caused by several structural components having differing natural frequencies, and thus reacting dissimilarly to acceleration. The NASTRAN program is quite useful in this regard because it provides for the creation of a file for each component, with each file having the same set of subcases for the desired acceleration states. The resultant force acting on each grid point is then obtained by superimposing the respective forces generated by each component, for each subcase.
Although structural analysis programs such as NASTRAN have proven to be invaluable in the structural design process, substantial calculations in addition to the computer-aided analysis must nonetheless be performed because structures typically are not modeled with the extreme amount of detail required to determine forces in individual fasteners. Furthermore, the computer programs of the prior art are unable to screen the numerous fastener forces for each subcase to determine which of such forces is the highest for each joint; or to deternmine which of the fasteners is transmitting the highest force, and thus is the most likely to fail.
In the absence of a computer program, engineers have calculated joint forces and designed fasteners for such joints using simplistic approaches which rely upon gross approximations. These prior art methods are time consuming, especially when the structure includes a large number of fasteners. Furthermore, the prior art methods typically result in conservative joint designs which most likely are stronger and thus weigh more than would be required by a more accurate analysis. On a structure including a large number of fasteners, the weight added by the inaccuracy of the prior art method is significant. When the structure is part of a propelled vehicle, such unnecessary weight degrades performance, decreases the payload, and increases fuel consumption.
The present invention is a computer program which, when used in conjunction with a compatible digital computer and the output of a structural analysis program such as NASTRAN, provides an accurate analysis of the axial and shear forces acting on the fasteners composing the joints of load-bearing members in a structure. It is substantially faster and more accurate than the simplistic methods of the prior art. It thus reduces the time necessary for analysis, while improving the reliability and reducing the weight of each fastener and its associated structure. Use of the present invention will thus provide a savings in the development cost of a structure, while also reducing its weight and improving its reliability.